Chemical Composition of RAW Materials | Construction Materials Part II

• The three basic constituents of hydraulic cement are lime, silica-alumina & iron oxide. 
• The relative proportions of these oxide compositions are responsible for influencing the various properties of cement. 
• An increase in lime content beyond a certain value makes it difficult to combine completely with other compounds. 
• Consequently, free lime will exist in the clinker & will result in any unsound cement. An increase in silica content at the expense of alumina & ferric oxide makes the cement difficult to fuse & form clinker. 
• The approximate limits of chemical composition in cement are given below: 

CONSTITUENTS OF PORTLAND CEMENT RAW MATERIALS

i). Lime, CaO: 

Function: Controls strength & soundness. Its deficiency reduces strength & setting time. 

Composition: 60 to 65 %

Average composition: 63%

ii). Silica, SiO2:

Function: Gives strength. Excess of it causes slow setting. 

Composition: 17 to 25%

Average composition: 20%

iii). Alumina, Al2O3:

Function: Responsible for quick setting, if in excess, it lowers the strength.

Composition: 3 to 8%

Average composition: 6%

iv). Iron oxide, Fe2O3:

Function: Gives color & helps in the fusion of different ingredients. 

Composition: 0.5 to 6%

Average composition: 3%

v). Magnesia, MgO:

Function: Imparts color & hardness. If in excess, it causes cracks in mortar & concrete and unsound. 

Composition: 0.5 to 4%

Average composition: 2%

vi). Soda & Potash, Na2O+K2O:

Function: These are residue, and if in excess cause efflorescence & cracking. 

Composition: 0.5 to 1%

Average composition: 1%

vii). Sulfur trioxide, SO3:

Function: Make cement sound. 

Composition: 1-2%

Average composition: 1.5%

IMPORTANT POINTS: 

• The rate of the setting of cement paste is controlled by regulating the ratio SiO2/(Al2O3 + Fe2O3).

• When development of heat of hydration is undesirable, the silica content is increased to about 21% & the alumina and iron oxide contents are limited to 6% each. 

• Resistance to the action of sulfate waters is increased by raising further the silica content 24% & reducing the alumina & iron contents in 4% each. 

• The variation in the composition depends largely on the ratio of CaO to SiO2 in the raw materials. 

• Alkalis accelerate the setting of cement paste.